Why do they insist on this as a feature? Isn't passive PoE not really that common outside of MT devices?
I think Passive PoE is more something that MikroTik introduced early on and haven't moved away from (yet). As for Passive PoE (PPoE - my abreviation, not anyone elses) not being common outside of MikroTik: That is much truer now than it used to be but other suppliers that use/d it include Motorola and Ubiquiti. PPoE was used in a lot of older equipment including WAPs, VoIP phones, IP cameras, Access control systems etc that (generally) didn't need anything more than a 100Mb link.
If you'd like some more information, take a look at the PoE Wikipedia page